In continental arc settings, the build up of dense magmatic roots, such as those exposed in southern Sierra Nevada batholith \citep*{Ducea_2001} and the Kohistan terrane \citep*{Jagoutz_2012}, represents another potential trigger for RTI \citep*{Jagoutz_2013}. In this case, multiple drips separated by a characteristic wavelength would not be expected in the arc-normal direction; rather, the entire width of the dense body should founder together. This foundering could take the form of delamination \citep{Wang2015a,Kay1993}, RTI dripping \citep{Wang2015b,Zandt2004a}, or an intermediate style characterized by peeling away (like delamination) of a dense body with high internal strain (like dripping) \citep{Beall2017}. In any case, the width of the instability should approximate the width of the dense root \citep{Wang2015}. If the magmatic root is elongated along strike, it is possible for multiple RTI drips to develop along strike; the spacing between the drips should correspond to that from linear stability analysis. A typical arc batholith is composed of ~30 km thick sequence of felsic volcanic and plutonic bodies overlying a mafic-ultramafic root. The root may be as thick as 90 km and prone to removal \citep*{Saleeby_2003}. For a 90 km thick layer, linear stability analysis predicts spacing of ~120 km between drips, though this distance would be increased by any mantle lithosphere that participates in removal, and by any decoupling of the root with the overlying batholith. Along-arc spacing between drips of 500 km or more is possible \citep{Harig2008}.
Other modes of foundering
- Foundering of a dense root: It's similar to RTI but doesn't fit the simple scheme above. Instead of 2 layers with an interface, there is a third body imposed within or between the layers.
- Weakening by fluids or melt and breaking off? (Northern Andean volcanic zone)
- Ablative removal, thermal erosion, shear entrainment, subduction removal, ... ?
Models - keep it brief
- Commonalities, setups, equations (Stokes), solvers, etc.
- Differences
- Insights: dripping itself, surface deformation, melting/volcanism, geochem (?)
- 2d vs 3d: future models should be 3d
Synthesis
- Scale of RTI should be ~150 km, larger if b/g strain rate is high
- Scale of other modes: smaller for weakened foundering, larger for root removal?
- Time scales?
- Volcanism and sources of melt - melting of drips possible
- Can we distinguish between RTI, weakened foundering, and root removal? If so, that would be a major part of the rest of the paper.
In summary, two triggers exist for RTI dripping. The first is a perturbation of the interface between the dense layer and the underlying asthenosphere. Such perturbations should evolve into drips spaced 100 to 200 km apart. Spacings of <50 km are possible if thermal diffusion of the perturbation is negligible; spacings of >200 km are possible if the Moho is approximately a stress-free surface. The second trigger involves the build up of a dense magmatic root. The entire width of the root should founder at once, resulting in an RTI drip with size approximately equal to the width of the root. If the root is elongated along strike, multiple such drips could develop, again with 100 to 200 km spacing.
Locations of dripping
North America
- Transverse ranges - first
- Southern SN (Isabella) - most papers, most data. Also northern SN (Redding)
- Wallowa mountains
- Central NV
- Grand Canyon
- Others
- Alternative hypothesis - fossil slabs
Western North America records the growth, evolution, and collapse of a continental-scale cordilleran orogen since Jurassic time \citep*{DeCelles_2004}. The presently extended and exposed nature of this taphrogen \citep*{Dickinson_2002} lends itself well to observing evidence of lithosphere removal events. On the other hand, confounding factors such as the continent-scale Farallon flat slab event \citep{BIRD_1988,Coney_1977} complicate interpretation of these observations.
Southern Sierra Nevada
Topographic subsidence
The Sierra Nevada range is a ~600 km long section of exhumed batholith bounded on the east by the extensional Basin and Range province and on the west by the Central Valley, a former forearc basin. Along much of the eastern Central Valley, pre-Holocene, west-dipping sedimentary strata are exposed at the surface \citep*{UNRUH_1991}. Between 37 and 36 N, however, the boundary between crystalline basement rocks and Cenozoic sedimentary strata bends eastward, and Holocene deposits directly onlap the Sierran basement rocks \citep*{Saleeby2004}. In this region of the Central Valley, steep basement rocks outcrop abruptly amid Holocene surficial deposits, and the rivers draining the Sierra flow into the endorrheic Tulare Lake basin. As inferred by \citet*{Saleeby2004}, Tulare basin sedimentation began ~2.2 Ma and extends ~40 km eastward in the subsurface. These observations can be explained by Pliocene-Holocene subsidence of the southern Central Valley and western Sierra foothills. Taking the Tulare Lake basin deposits together with the area of Holocene onlap, subsidence occupies a circular region approximately 100 km in diameter. Subsidence appears to be concentrated at the modern Tulare Lake, though this could also represent large scale tilting superimposed on smaller scale dynamics.
Topographic uplift
The uplift history of the Sierra Nevada is disputed. Apatite (U-Th)/He dating suggests that the central Sierra Nevada had obtained 4.5 km of mean elevation and 3 km of relief by late Cretaceous time, and that mean elevation and relief have decreased to present values since 70 Ma \citep*{House_1998}. Cenozoic uplift has been long interpreted as a uniform, westward tilting of the Sierra Nevada. Paleochannels preserved by lahar flows appear to be tilted by up to 25 m/km in the western Sierra foothills. By extrapolating this tilt linearly to the range crest, \citet*{Huber1990} argued for 2 km of surface uplift since 20 Ma, 1 km of which occurred in the last 3 Ma. Such surface uplift, however, would only occur in the absence of erosion, when rock uplift equals surface uplift; river profile tilting near the hinge line is also consistent with flexural loading (unloading) of the Central Valley (Sierra Nevada) due to deposition (exhumation), which could account for most, if not all all, of the observed tilting \citep*{Small_1995}. Flexural unloading due to exhumation lowers mean elevation and is driven by changes in climate, such as the onset of Pliocene glaciation \citep*{England_1990}. The recognition that erosion could be responsible for a large portion of the observed rock uplift in the Sierra, and that high topography existed before 3 Ma, is supported by other lines of evidence. The isotopic compositions of hydrated volcanic glasses in the Basin and Range imply the existence of a rain shadow similar to the modern one since at least 12 Ma \citep[see][]{Mulch_2008}; numeric models of river incision suggest ~0.5 km of surface uplift around 10 Ma \citep*{Pelletier_2007}; and paleoelevation proxies indicate high elevations in the Eocene \citep*{Mix_2015}. These studies indicate that, although substantial rock uplift has occurred since 10 Ma, the Sierra Nevada have been high since the late Cretaceous. Some combination of climate and geodynamic processes probably resulted in a pulse of late Miocene-Pliocene uplift, but the contribution of geodynamics to this pulse is not well constrained. Here we recognize the ongoing research on this topic and consider it probable that the southern Sierra Nevada experienced some tectonic uplift in the late Cenozoic, around 10-3 Ma, though the magnitude may be on the order of 1 km or less.
Volcanism
Xenoliths
Good evidence for removal
Tomography
Red herring
South America
Pamir-Himalaya-Tibet
- Pamir - Vanj and Dunkeldik
- Himalaya
- Tibet
Other
- SE Carpathians?
- New Zealand
Synthesis